Ocean Measurementsfrom Space in 2025

OVERVIEW

Ocean measurements from space have advanced significantly since the first sensors were flown on NASA satellites such as Seasat in the 1970s. New technologies have opened the door to new, unforeseen scientific questions and practical applications, which, in turn, have guided the next generation of technology development—a fruitful, mutual coupling between science and technology. Recent advances in modeling of ocean circulation and biochemistry are now also linked to improved measurement capabilities.

Since the ocean is largely opaque over much of the usable electromagnetic spectrum, ocean measurements from space are largely confined to surface properties such as SSH, SST, surface wind vectors, sea surface salinity (SSS), ocean color, and surface currents. In some cases properties of the ocean beneath the surface can be inferred from such measurements, the most striking example being the determination of bathymetry from sea surface height measurements made by altimeters. Measurements of variations in the Earth’s gravity fields (e.g., by NASA’s Gravity Recovery and Climate Experiment [GRACE]) mission are somewhat a special case, and have been used to infer ocean bottom pressure, for example.

With the release of the 2007 decadal survey for Earth Science and

*

California Institute of Technology

The National Academies of Sciences, Engineering, and Medicine 500 Fifth St. N.W. | Washington, D.C. 20001

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Glickson, Deborah "Ocean Measurements from Space in 2025--A. Freeman."
Oceanography in 2025: Proceedings of a Workshop.
Washington, DC: The National Academies Press, 2009.

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Ocean Measurements
from Space in 2025
A. Freeman*
OvERvIEW
Ocean measurements from space have advanced significantly since
the first sensors were flown on NASA satellites such as Seasat in the
1970s. New technologies have opened the door to new, unforeseen scien-
tific questions and practical applications, which, in turn, have guided the
next generation of technology development—a fruitful, mutual coupling
between science and technology. Recent advances in modeling of ocean
circulation and biochemistry are now also linked to improved measure-
ment capabilities.
Since the ocean is largely opaque over much of the usable electro-
magnetic spectrum, ocean measurements from space are largely confined
to surface properties such as SSH, SST, surface wind vectors, sea surface
salinity (SSS), ocean color, and surface currents. In some cases properties
of the ocean beneath the surface can be inferred from such measurements,
the most striking example being the determination of bathymetry from
sea surface height measurements made by altimeters. Measurements of
variations in the Earth’s gravity fields (e.g., by NASA’s Gravity Recovery
and Climate Experiment [GRACE]) mission are somewhat a special case,
and have been used to infer ocean bottom pressure, for example.
With the release of the 2007 decadal survey for Earth Science and
* California Institute of Technology
92

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93
A. FREEMAN
Applications from Space (NRC 2007), we are poised on the brink of a
series of improvements in ocean measurements from space that will revo-
lutionize oceanography from space in the next decade—as big an advance
or bigger than the advent of ocean altimetry with TOPEX/Poseidon. This
white paper looks forward to that timeframe and beyond, towards the
type of measurements we should expect in 2025, and the science questions
that we should be able to ask.
OvERARCHING SCIENCE qUESTIONS
The scientific and practical questions that are likely to drive develop-
ments in the 2025 timeframe are:
• Oceans as part of the coupled ocean-atmosphere-ice-land-
biogeochemical system
– Most current ocean models that assimilate data are presently
run in ‘forced’ mode; they do not affect the atmosphere. Most
atmospheric models that assimilate data use an overly sim-
plified representation of the oceans (a mixed layer) or worse,
only sea surface temperature. These are due to computational
expense, a barrier that is fast receding.
– CO2 uptake, hurricanes, ENSO, ice shelf disintegration and ice
sheet advance are all examples where the coupling between
the ocean and in these cases either the atmosphere or the cryo-
sphere are critical.
• Description and prediction of the global water cycle in the con-
text of global climate change can only be fully realized when the
marine branch of the hydrological cycle is considered.
• Increased spatial and temporal resolution in ocean observations,
ocean models, and climate models.
– Spin up / spin down time scales in the oceans depend on eddy
(~ 100 km or less) parameterization. These time scales are essen-
tial for climate forecasts. Thus climate models need to resolve or
parameterize properly ocean eddies for realistic climate simula-
tions (Marshall, personal communication, 2008).
– In ocean models, dissipation of momentum is achieved through
enhanced vertical viscosities and drag laws with little physical
validation. Turbulent transport of tracers like heat, salt, carbon
and nutrients is represented with unphysical constant eddy
diffusivities in numerical ocean models. Ocean models run-
ning at sufficient resolutions to address submesoscale (1-100
km) dynamics have just begun to emerge (Capet et al. 2008).

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9 OCEANOGRAPHY IN 2025
Global observations at these scales are needed to constrain the
models.
– For coastal work, forecasting for navigation, inundation, and
marine resources critically depends on short length scales, con-
trolled by the shallow ocean depth.
• Need to forecast with increasing accuracy and shorter time delays
both short time scales (navigation, harmful algal blooms) and
long ones (climate) for societal benefit.
To address these questions, we believe a progressive improvement in
measurement capability is necessary, across a broad range of parameters,
as outlined in Table 1 and the discussion below.
TECHNOLOGICAL ADvANCES
The kind of technology advances that will enable the improvements
in ocean measurements from space described above include:
• Miniaturized, more efficient radar components to reduce mass/
power needs of radar electronics
• Efficient, high-power transmitters at shorter wavelengths (espe-
cially Ku- and Ka-Band)
• Increased onboard processing and/or downlink capability, allow-
ing data acquisition at higher spatial and temporal resolution.
• Larger deployable antennas in the 6-12 m range, particularly
employed in a conical scan mode, enabling higher resolution
radiometry and scatterometry.
• A scanning interferometer pair of antennas, rotating through an
azimuth scan of 360 degrees to provide along-track interferom-
etry measurements of surface currents at high resolution.
• Precision formation flying, to enable bistatic wide-swath sea sur-
face height measurements from two platforms flying in forma-
tion, and gravity measurements from multiple platforms.
• Laser interferometry to improve the accuracy of gravity measure-
ments from future GRACE-like missions.
• Wide field of view imaging spectrometers with improved stability
and signal to noise (SNR) and atmospheric correction capabili-
ties, enabling global ocean biosphere measurements at moderate
resolution (~1 km) on a daily basis and on fine resolution (60 m)
on synoptic basis.
• Deployment of ocean color imagers on geostationary platforms
to sample the highly dynamic processes of coastal ecosystems.
• The spaceborne implementation of active remote sensing of bio-

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9
A. FREEMAN
chemical constituents of the ocean, including fluorescence spec-
troscopy instruments (UV/visible) and lasers at the blue end of
the visible spectrum to measure mixed layer depth, as input to
biochemical models.
• Increased computational power will allow more complex coupled
models to be run at higher resolutions, and data assimilative
models to assimilate data.
ACKNOWLEDGEMENTS
The work described in this paper was, in part, carried out by the
Jet Propulsion Laboratory, California Institute of Technology, under a
contract with the National Aeronautics and Space Administration. Spe-
cial thanks to V. Zlotnicki, T. Liu, L-L. Fu, B. Holt, R. Kwok, S. Yueh, I.
Fukumori, J. Vazquez, D. Siegel, and G. Lagerloef for contributing to this
white paper.
REFERENCES
NRC. 2007. Earth Science and Applications from Space: National Imperaties for the Next Decade
and Beyond. National Academies Press, Washington, D.C.
Capet, X., J.C. McWilliams, M.J. Molemaker, and A.F. Shchepetkin. 2008. Mesoscale to
Submesoscale Transition in the California Current System. Part II: Frontal Processes.
Journal of Physical Oceanography. 38: 44-64.